Hollow shafts of a large diameter, for example, a diameter of more than one meter, are needed as rotors for some special applications. An example for such an application is a wind turbine, where such a hollow shaft is usually used as a rotor yoke for a wind turbine generator, for example a direct drive wind turbine generator.
Such a rotor yoke of a generator for a large scale modern wind turbine can be built up of a cylinder of rolled steel, having a steel thickness of about 40 mm and a diameter of for example, four meters for an outer rotor type generator. Rotors for inner rotor type generators are somewhat smaller in diameter.
The manufacture of such a rotor yoke must take into account some specifications, for example certain textures of the inner and the outer surface, in particular for receiving magnets, and demanding requirements regarding the tolerance of the diameter, which can e.g. be in the order of plus/minus 0.5 mm from the desired diameter.
To manufacture such a rotor, it is known to process a hollow cylinder using machining tools, for example a specialized lathe for multiple machining process steps, until the desired low tolerance of the rotor is achieved. These processing steps are time consuming and expensive steps of the manufacturing process. Consequently, such generators, in particular direct drive generators, are seldom used, despite the other advantages this kind of generator has.